1 Cor 15:3-11
3 For I delivered to you as of first importance what I also received, that Christ died for our sins in accordance with the scriptures, 4 that he was buried, that he was raised on the third day in accordance with the scriptures, 5 and that he appeared to Cephas, then to the twelve. 6 Then he appeared to more than five hundred brethren at one time, most of whom are still alive, though some have fallen asleep. 7 Then he appeared to James, then to all the apostles. 8 Last of all, as to one untimely born, he appeared also to me. 9 For I am the least of the apostles, unfit to be called an apostle, because I persecuted the church of God. 10 But by the grace of God I am what I am, and his grace toward me was not in vain. On the contrary, I worked harder than any of them, though it was not I, but the grace of God which is with me. 11 Whether then it was I or they, so we preach and so you believed.
This passage has plenty to “debunk” that theory all by itself. Paul himself states that he received the gospel. He faithfully passed on what was taught to him. He mentions the Apostles to whom the Gospel was taught by Jesus, and admits that he persecuted the Church (which was already in existence before he was converted)
I am sure there is more, but this passage will take a lot of explaining.
Also, the believers were first called Christians in Antioch. It may be this did not occur until Paul was brought there to teach.
25 So Barnabas went to Tarsus to look for Saul; 26 and when he had found him, he brought him to Antioch. For a whole year they met with the church, and taught a large company of people; and in Antioch the disciples were for the first time called Christians.
However it is clear that the church was already in existence in Antioch before he arrived.